1. Field of the Invention
The present invention relates to an optical element such as an objective lens, which is mounted in an optical head for an optical pick-up apparatus, a concerned objective lens, lens unit composed of a plurality of optical elements, lens frame for the lens unit, positioning method of the lens unit, and an optical pick-up apparatus.
2. Description of the Related Art
Conventionally, each kind of optical pick-up apparatus for reproducing and recording of the information for an optical information recording medium such as CD (Compact Disk), DVD (Digital Video Disk) is developed, manufactured and spread in the market. As an optical head apparatus mounted in such an optical pick-up apparatus, there is an apparatus in which an objective lens main body is fixed to a holder with a phase control element and they are integrated in order to stably record and reproduce both of CD and DVD. In this case, positioning marks for putting the central axis together to the objective lens main body and the phase control element are respectively provided and de-centering is prevented, and the reduction of the wave-front aberration is attained. (Refer to Patent Document 1).
(Patent Document 1) Tokkai No. 2001-6203
However, in the de-centering prevention method of the optical head as described above, a positioning of the objective lens main body and the phase control element is premised on the assumption that both are arranged in very close positions, and as both are separated, the accuracy of the positioning is lowered. Further, in the above-described de-centering prevention method, it is a basic premise that the optical axis of the objective lens main body and the optical axis of the phase control element are held in parallel with each other, and when both are inclined, there is a possibility that the accuracy of the positioning is lowered or the aberration is generated.
Further, recently, there is also a case where the objective lens main body and the phase control element are mounted in an optical head apparatus as a lens unit in which the objective lens main body and the phase control element are integrated by direct or indirect cementation. When the imaging characteristic is measured for such a lens unit, for example, the measurement by using an interferometer is conducted, however, there is a case where, in this kind of measurement, the interference fringe is disturbed by the returning light from the phase control element or its contrast is lowered, and the measurement accuracy is lowered.
Further, also in the case where the optical pick-up apparatus in which such a lens unit is mounted, is used, there is a possibility that the returning light from the phase control element is incident on the photo detector, and the bad influence is affected in the same manner as described above.
Further, recently, BD (Blue-ray) or HD (HD DVD), which are made further high-density, series optical pick-up apparatus is developed, and there is a needs for an objective lens of compatible type corresponding to DVD and BD, or DVD and HD, or 3-wavelength compatible type objective lens corresponding to CD, DVD and BD, or CD, DVD and HD. In this kind of objective lens, generally, in order to have a structure to give a compatibility for CD, DVD, or CD and DVD, by the phase control element, to the objective lens main body whose aberration is corrected for BD or HD, the position or inclination of the phase control element to the objective lens main body, particularly, because the shift de-centering for the objective lens main body of the phase structure which is formed in the phase control element, affects the important influence on the imaging characteristic of DVD series or CD series on the long wavelength side, the accuracy of several micron meter is required for the positioning of the phase control element.